This disclosure relates generally to the field of in-flight radiometric calibration, and in particular to a ground based specular array for radiometric calibration by providing calibration targets with known intensity, traceable to the solar spectral constant.
Airborne, and space born remote sensing systems for geospatial survey and observation are increasing in use and application. Such systems may be employed to detect anthropogenetic and natural effects in climate change, geologic morphology and chemistry, hydrology, vegetation health, as well as to identify and distinguish between friend and foe military targets, drug operations, terrorist activities, damage assessment, and emergency routes for responding to natural disasters.
Achieving these capabilities demand quantitative links be established between the image data and the physical properties and processes the sensor is trying to observe. That is, ideally the sensor should be capable of performing as an absolute calibrated radiometer. Thus, system providers strive to provide adequate methods for addressing stability and accuracy requirements imposed by the user community to define and validate sensor radiometric performance and in turn establish the level of confidence achieved in data exploitation.
Development of monochromatic and multispectral sensing systems continue to move toward increasing spatial resolution in response to the fact that most targets of interest are contained in only a few pixels or even subpixel. Generally, each image is composed of a plurality of pixels, with the sensed reflectivity of light from each pixel analyzed to determine the physical content and make up of the target contained in a pixel area. However, for small targets, the blur spot; due to optical diffraction, electronic readout, sensor motion, atmospheric scattering and/or combinations thereof as well as other potential issues, smears light into nearby pixels spatially disconnected from the target. Indeed, multispectral and hyperspectral sensors collect image data across dozens if not hundreds of spectral bands in which the blurring effect varies because it is wavelength dependent. As a result, knowledge of the spatial performance (i.e. sensor point spread function) must be applied as part of the radiometric calibration process if effective small target radiometry will be achieved If done, analysis of the contiguous spectrum will permit finer spatial resolution and improved radiometric perception of information contained in the image then is otherwise apparent if one ignores the effects of sensor blurring on small targets.
A critical element in the operation of airborne and space borne imaging systems is sensor calibration on the ground before launch or flight. However, it is entirely possible that physical conditions within the imaging system or in the atmosphere between the imaging system and the desired target may change from the calibration laboratory setting in such a way so as to skew the calibration values. The calibration performance thus becomes suspect until validated after deployment. Indeed, to assure the most accurate operation and sensing absolute calibration is most desired.
Absolute calibration is achievable when the sensor system images a calibration source that provides a known radiant flux at the input aperture of the optical system in units of intensity (watts/str), irradiance (watts/m2), or radiance (watts/m2/str). This provides the information needed to establish calibration coefficients converting pixel digital numbers (DN) to at-sensor radiance On-board calibration systems in the solar reflective spectrum have been employed in many instances, however they are generally complex, expensive and taxing on mass limitations if absolute calibration is to be delivered over the life of the mission.
Prior art on-board calibration systems for delivering a known radiant flux in the solar reflective spectrum are known, but succumb to limitations in performance. There are two main categories delivering on-board capabilities: 1) Internal sources and 2) solar diffusers. Internal sources consist of lamps and/or high temperature blackbodies as the calibration reference. Technical shortcomings include inability to match the color temperature of the sun, degradation in radiant output over time, and inability to illuminate (and thus monitor) the entire optical system for contamination and degradation effects.
Solar diffusers have the advantage that they reproduce the spectrum of the sun and can propagate light through the full sensor optical system. However, solar diffusers still suffer from bi-directional reflectance effects and degrade with time. For high spatial resolution systems (requiring large optics) they provide only partial aperture illumination and thus are unable to match the full light path illumination of the focal plane compared to operational targets, creating image artifacts after calibration.
It is also noted that these on-board calibration systems are designed to be extended sources filling a large portion if not all the sensor field-of-view. As a result, blurring effects are averaged out making them ineffective in addressing image quality and spatial issues related to small target radiometric calibration. Further still, an on-board calibration system is capable of calibrating only the immediate system to which it is coupled. Thus, for a sensor constellation there is a potential cost savings through simpler hardware design and reduced laboratory testing, if multiple imaging systems could enjoy use of the same calibration system. In addition, an off-board calibration source supporting inter-sensor calibration, can reveal sensor to sensor bias errors undetectable by an individualized on-board calibrator. This capability is achieved through vicarious calibration methods that image celestial or ground based targets of known radiometric properties.
The Moon and stars have found use in prior art for the calibration of space based systems. The Moon provides an extended source that is limited in access each month and has proven to be problematic in achieving accurate absolute calibration, Stars are more available but typically have spectral properties that are significantly different from the sun and generally support calibration in only the lower end of the sensor operational dynamic range. For airborne systems, neither of these targets is practical for calibration. Thus, they cannot support inter-sensor calibration between airborne and space-based assets.
Current ground based vicarious calibration methods generally involve large surfaces of knowable reflectance, either natural (desert dry lake bed playa or uniform grass fields) or man-made (tarps or diffuse panels). Natural targets have an unstable reflectance with significant bi-directional effects and generally provide only one light flux level for each calibration collect. Man-made diffuse reflectance targets provide better control of reflectance properties and multiple flux levels but, in order to be useful, must still be large, filling many pixels (typically on the scale of twenty to fifty meters or more) They are therefore cumbersome to set out requiring an extensive support team for deployment and maintenance. Both techniques require a broad range of ground truth measurements that characterize target and atmospheric optical properties at the time of the overpass for radiometric calibration.
Hence there is a need for a system and method of radiometric calibration with new capabilities that overcomes one or more of the technical problems noted above.